(1) Field of the Invention
The present invention relates to a system and method for controlling the air-fuel mixture ratio of an air-fuel mixture supplied to an internal combustion engine, without using a conventional sensor for directly detecting intake air quantity.
(2) Background of the Art
Generally, an air-fuel mixture ratio of an air-fuel mixture supplied to an engine is controlled in such a way that an intake air quantity is measured by means of one of various kinds of intake quantity measuring means such as a vane-type, hot-wire type, or Karman vortex type intake air quantity sensors and a quantity of fuel supplied to the engine is determined on the basis of the measured air quantity.
The vane-type intake air quantity sensor is such that a quantity of rotation of a vane rotating in response to an intake air pressure applied thereto is detected by means of a potentiometer so as to measure the intake air quantity.
The hot-wire type intake air quantity sensor is such that a current flowing through a bridge circuit is controlled on the basis of a change in a resistance value of a hot-wire resistor according to the intake air quantity and the controlled current value is used to detect the intake air quantity.
The Karman vortex intake air quantity sensor is such that a quantity of vortex downstream of a probe thereof is measured to detect the corresponding air quantity.
However, in the air-fuel mixture ratio controlling system in which the air-fuel mixture ratio is controlled on the basis of the detected intake air quantity, one of the above-described intake air quantity detecting means is needed since such intake air quantity detecting means occupies a high percentage of cost with respect to a whole engine cost, the cost of installing such an intake air quantity detecting means on the engine is very high and is less reliable when a low-cost engine is developed or when the highly reliable whole engine controlling system is installed with fewer numbers of parts.
A Japanese Patent Application First Publication Sho 61-55349 published on Mar. 19, 1986, exemplifies one of the air-fuel mixture ratio controlling systems in which an internal cylinder pressure and a load imposed on the engine (fuel injection quantity) are detected and the air-fuel mixture is controlled on the basis of the detected values of the internal cylinder pressure and engine load so as to bring a ratio of internal cylinder pressure to an engine load into an ideal value. In the air-fuel mixture ratio controlling system disclosed in the above-identified Japanese Patent Application Publication, the intake air quantity detecting means can be omitted.
However, it is difficult to estimate the air-fuel mixture only on the basis of the internal cylinder pressure since, as a matter of fact, a waveform of the internal cylinder pressure is complicated varied.
Therefore, the accuracy of controlling the air-fuel mixture ratio cannot be improved further.